In the existing communication system, a network centralized control scheme is adopted, thus transmissions of both uplink and downlink data of a User Equipment (UE, also referred to as a terminal) are controlled by the network side. Communication between two user equipments needs to be forwarded and controlled by a core network.
In the course of the evolvement of the communication system, in order to better satisfy the demand of a user and to improve the efficiency of exchanging information, the mechanism of a discovery and even direct communication between the user equipments, i.e., Device to Device (D2D) communication, has been introduced.
If a demand for D2D communication between two user equipments close to each other is satisfied, then the two user equipments can conduct D2D communication with each other. There are the following two scenarios in this situation.
In a first scenario, one of the user equipments initiates a service request for the other user equipment to the network side, and the network side instructs these two user equipments to establish a D2D link, upon determining that the demand for D2D communication between the two user equipments is satisfied.
In a second scenario, the network side instructs these two user equipments to switch the communication from a Device to Network (D2N) link to a D2D link, upon determining that the demand for D2D communication between the two user equipments communicating with each other over the D2N link is satisfied.
If the two UEs in D2D communication go far away from each other due to their movements so that the demand for D2D communication is not satisfied, then the communication between these two user equipments need to be switched from the D2D link to the D2N link.
However there has been neither a particular solution to switching between the D2D link and the D2N link, nor a particular solution to establishing the D2D link.